Communications devices such as modems, cable modems, digital subscriber line (DSL) modems and the like are becoming increasingly common. Such devices typically facilitate data communications between a source and a destination across a communications medium such as a telephone line, coaxial cable, twisted pair cabling, copper wires, fiber optics, radio frequency (RF), infrared or other wireless interface, or the like. Generally, such communications are bi-directional in that both source and destination are allowed to transmit and receive data over the same medium.
With reference to FIG. 1, a conventional communications system 100 includes a communications device 102 (such as a modem) that handles transmit data 112 and receive data 114 on a communications medium 104 (such as a telephone line). Typically, transmit data 112 is provided to the medium 104 via a line driver 150. Line driver 150 is commonly represented as an amplifier 106 and an output impedance Z.sub.LD (represented in FIG. 1 as resistor 108). Receive data 114 may be provided from medium 104 to communications device 102 via, for example, a separate data path 110 that bypasses amplifier 106.
Generally speaking, it is desirable to match the total output impedance of the line driver to the impedance of communications medium 104 (represented in FIG. 1 as Z.sub.line). The total output of the impedance may be represented as the series of the line driver output impedance (which is usually quite low, such as on the order of a few milli-ohms) with line driver impedance Z.sub.LD. Communications medium 104 typically exhibits a relatively low impedance (e.g. on the order of 100 ohms), so Z.sub.LD may be designed to be correspondingly low (e.g. on the order of 100 ohms, or as otherwise appropriate) when the line driver is activated. Failure to at least approximately match Z.sub.LD to Z.sub.line could result in undesirable reflections in medium 104, thus resulting in unwanted noise.
Further, it is generally desirable to conserve power in line driver 150 whenever possible. One technique for reducing overall power consumption involves powering down line driver 150 when device 102 is not transmitting on medium 104. For reasons that will become apparent, it has been generally difficult to power down line driver 150 without affecting impedance Z.sub.LD. Hence, the driver impedance Z.sub.LD of many prior art systems did not adequately match the impedance Z.sub.line of communications medium 104 during power-down mode even if the impedance's properly matched when the line driver was powered up.
It is therefore desirable to create a line driver that provides adequate impedance matching with the communications line during all modes of operation while reducing overall power consumption.